Add "units" attr to the coords object and dataarray.

[arafune]

Especially, we would like to use both "degrees" and "radians" in the coordinates, with the ability to switch between them being ideal. And current version we can use both.
Some data use "counts" as the unit for intensity, while others use "cps."
Furthermore, I currently use "fs" as the unit for "delay," but we might need to switch to "ps" instead.

Using Pint might be a comprehensive solution, but it could be challenging to maintain and use for many users.
I think the most practical approach is to add "units" attributes to the Coords and DataArray objects.

In line with this, the switching_angle_units function should be refactored.